Titanium dioxide (TiO.sub.2 : hereafter, sometimes referred to as titanium oxide for brevity) is known as a white pigment, ultraviolet light screening agent or the like, and is utilized widely as a coloring agent or an opacifying agent for plastics articles. Additives such as titanium dioxide and fillers are kneaded into plastics in heated and molten state. Hitherto, polyolefin-based plastics used as a plastic film, sheet or the like are kneaded at a temperature around 200.degree. C., which temperature is lower than the temperature at which engineering plastics are processed. Recently, however, there is a tendency that in order to increase productivity, height temperatures elevated to about 300.degree. C. are used for mixing, kneading and molding so that process time can be reduced. Also, the requirement for the precision of the trimmed shape of articles is becoming stricter. This leads to severer requirement for the physical properties of titanium dioxide powder to be mixed in plastics.
More specifically, titanium dioxide powder is demanded which is easy to disperse in plastics, has high opacifying power, is excellent in weatherability and light resistance, and generates less amount of a gas or gases upon processing. Although titanium dioxide powders are not influenced remarkably by gas generation at kneading temperatures around 200.degree. C., above which temperature the organic components in the surface treating agent used for surface modification of titanium dioxide will decompose or gaseous components will vaporize such as liberation of water of crystallization to form foams, so that there occur frequently problems of causing failure of quality.
When thermoplastic resins are pigmented with the above-mentioned titanium oxide, there is used a pigmenting composition. As the pigmenting composition, there have heretofore been used dry colors in the form of powder which comprise a pigment and a dispersant, liquid colors or paste colors which comprise a pigment dispersed in a dispersant that is liquid at ambient temperature, masterbatches which comprise a resin that is solid at ambient temperature and a pigment dispersed therein in high concentrations, the mixture being shaped into pellets or flakes. Of these, frequently used are masterbatches because they are easy to handle and are excellent in maintaining operational environment upon use.
As described above, titanium oxide is used in large amounts as a white pigment for pigmenting resins because of its excellent whiteness, high opacifying properties, and high pigmenting power. However, titanium oxide by nature is hydrophilic and therefore is subjected to surface treatment with inorganic or organic substances in order to improve weatherability, hydrophobicity and dispersibility.
For example, there are known titanium oxides treated with polyol- and polysiloxane-based surface treating agents, respectively. The titanium oxide whose surface is treated with a polyol-based surface treating agent is highly dispersible but it receives less effect of hydrophobic treatment so that it adsorbs much water. Masterbatches which contain such a surface-treated titanium oxide and a thermoplastic resin also are less hydrophobic so that they tend to contain much water.
On the other hand, titanium oxide whose surface is treated with a polysiloxane-based surface treating agent has a weak chemical bonding power between the surface of titanium oxide and the surface treating agent although it receives excellent hydrophobic effect so that the surface treating agent will peel off depending on the kneading conditions or extruding conditions when preparing the masterbatch by using these titanium oxide and a thermoplastic resin, which results in unevenness in the state of dispersion of titanium oxide in the masterbatch or an increase in the moisture content of the masterbatch because of a decrease in hydrophobicity.
When it is tried to prepare pigmented resin molded articles using a masterbatch with a high moisture content and a thermoplastic resin, the masterbatch and thermoplastic resin will slide in the extruding machine due to the moisture generated in the masterbatch. This causes a failure of dispersing or the moisture to return to the feeding port of the machine for feeding raw materials so that the raw materials cannot be fed smoothly. This is also a problem.
In particular, in the field of melt-extruded laminates which require high temperature processing at temperatures as high as 280 to 350.degree. C. at which elimination of water of crystallization will occur, moisture content is extremely important; a problem arises that if the moisture content is high, water evaporates when extruding a film and there occurs the phenomenon that reticulated pores are formed in the film, i.e., so-called lacing, so that the film cannot be put in practical use.
The present invention is to solve the above-described problems accompanying the conventional titanium dioxides and has an object of the present invention to provide titanium dioxide powder which has sufficient dispersibility and hydrophobicity and generates less evaporated water upon high temperature processing when kneaded into synthetic resin articles, and a manufacturing method therefor as well as to provide a masterbatch which generates less moisture upon high temperature processing.
Further, in a preferred embodiment, the present invention provides anatase-type titanium dioxide which generates less evaporated water and has a high color stability and a manufacturing method therefor as well as a masterbatch containing such an anatase-type titanium dioxide.